Hi,
as discussed before:
The root cause for the performance regression is the smp_mb() that was
added into the fast path.
I see two options:
1) switch to full spin_lock()/spin_unlock() for the rare codepath,
then the fast path doesn't need the smp_mb() anymore.
2) confirm that no arch needs the smp_mb(), then remove it.
- powerpc is ok after commit
6262db7c088b ("powerpc/spinlock: Fix spin_unlock_wait()")
- arm is ok after commit
d86b8da04dfa ("arm64: spinlock: serialise spin_unlock_wait against
concurrent lockers")
- for x86 is ok after commit
2c6100227116 ("locking/qspinlock: Fix spin_unlock_wait() some more")
- for the remaining SMP architectures, I don't have a status.
I would prefer the approach 1:
The memory ordering provided by spin_lock()/spin_unlock() is clear.
Thus:
Attached are patches for approach 1:
- Patch 1 replaces spin_unlock_wait() with spin_lock()/spin_unlock() and
removes all memory barriers that are then unnecessary.
- Patch 2 adds the hysteresis code: This makes the rare codepath
extremely rare.
It also corrects some wrong comments, e.g. regarding switching
from global lock to per-sem lock (we "must' switch, not we "can"
switch as written right now).
The patches passed stress-testing.
What do you think?
My initial idea was to aim for 4.10, then we have more time to decide.
--
Manfred
